1. Field of the Invention
The present invention generally relates to optical waveguide devices and in particular to an optical device capable of being optimized to function as a broad frequency band optical switch in one embodiment and a narrow pass-band optical filter in a second embodiment, both embodiments being capable of light intensity modulation and of being switched between parallel and cross states at switching frequencies ranging from dc to high microwave.
2. Description of the Prior Art
The development of optical communication systems, and in particular optical fiber communication systems, have progressed rapidly in the last few years. In these systems, the concept of integrated optics has been rapidly developed wherein the processing of an optical signal is accomplished by semiconductor integrated circuits. A typical system can comprise a semiconductor laser formed on a chip substrate, and various switches, filters, amplifiers and coupling elements all formed on the same chip. Many systems may also include single mode optical fibers which are coupled to channel waveguides formed on the chip substrate. Thus, several integrated optical switches and filters can be used on the same chip for flexibly multiplexing, switching, and demultiplexing different wavelengths in an optical systems, such as an optical fiber communication network.
A prior art technique for providing an optical filter and which is capable of operating as an optical multiplexer/demultiplexer is disclosed in U.S. Pat. No. 4,146,297 to Alferness et al. The Alferness et al patent discloses an integrated optical filter composed of non-identical strip waveguides in LiNbO.sub.3. The center filter frequency of the device is tunable over a wavelength difference of several hundred angstroms and electrical control of the crossover switching efficiency is provided. Although the optical filter described in Alferness et al performs very satisfactorily, the bandwidth, with respect to the crossover efficiency, is relatively broad and the sidelobe characteristic of the filter is larger than desired. What is desired therefore is to provide an optical filter generally of the type disclosed in the Alferness et al patent but with the bandwidth narrowed and the sidelobes reduced. This, in turn, will reduce the noise and optical cross talk level of the device, thus enhancing the optical filter characteristics of the device.
For integrated optical switches, there are several important operating parameters: switching voltage and power, optical cross talk, switching speed, and optical loss. If significant optical cross talk can be tolerated and high data rates (greater than 1 GHz) are required, then a simple directional coupler such as that disclosed in the article of Izutsu et al, "1 GHz Bandwidth Traveling-Wave LiNbO.sub.3 Optical Waveguide Modulator," IEEE Journal Quantum Electronics, Vol. QE-14, p.394, 1978, can be utilized with a traveling wave switch control signal source. However, if low cross talk (less than -20 db) is required, data rates must be reduced to less than one GHz and a .DELTA..beta. reversal switch, such as disclosed in the article of Kogelnik et al, "IEEE Journal Quantum Electronics, Vol. QE-12 p.396, 1976, must typically be used. U.S. Pat. No. 4,262,993 to Burns et al is illustrative of an alternating .DELTA..beta. switch having sets of electrodes and waveguides formed on a crystalline substrate, the substrate being formed in abutting sections. Although the abutting sections reduce the voltages required for switching, the fabrication process required is complex and relatively expensive. U.S. Pat. No. 4,012,113 to Kogelnik et al describes an optical switch provided with means for adjusting the crossover and parallel states. The driving electrodes may be split into two or more sequential sections to allow for electrical adjustability of the cross state. However, the switch described in the above patent is a modified .DELTA..beta. reversal switch and is thus similarly limited with respect to switching speeds, i.e, to less than approximately 1 GHz. It would be desired if an optical switch could be provided wherein low cross talk (comparable to the prior art .DELTA..beta. reversal switch) and high switching speeds, as is available on devices utilizing traveling wave interaction, could be provided.